The applicant of the present invention previously proposed a chip-type solid electrolytic capacitor as shown in FIG. 4 (see Japanese Unexamined Patent Publication No. HEI8-148392(1996)).
As shown in FIG. 4, the solid electrolytic capacitor 1 includes a capacitor element 2 having lead frames 9, 90 provided on a lower surface thereof. The capacitor element 2 is encapsulated in a housing 7 of a synthetic resin. The lead frames 9, 90 are each bent along two edges over peripheral surfaces of the housing 7. The capacitor element 2 includes an anode body 20 of a valve metal formed with a dielectric oxide film 21, and a laminate cathode 5 provided on the dielectric oxide film 21. The laminate cathode 5 includes a solid electrolyte layer 3, a carbon layer 6 and a silver paste layer 60. The valve metal herein means a metal to be formed with a highly dense and durable dielectric oxide film by electrolytic oxidation, and examples thereof include Al (aluminum), Ta (tantalum), Ti (titanium) and Nb (niobium). Exemplary solid electrolytes for the solid electrolyte layer 3 include electrically conductive polymers such as polythiophenes and polypyrroles.
The carbon layer 6 and the silver paste layer 60 are completely different in particle diameters, electrical characteristics and the like, so that a contact resistance occurs between the carbon layer 6 and the silver paste layer 60. This increases the ESR (equivalent series resistance) of the solid electrolytic capacitor 1. Therefore, a three-layer structure is proposed which includes a mixture layer of a mixture of carbon powder and silver powder provided between the carbon layer 6 and the silver paste layer 60 (see Japanese Unexamined Patent Publication No. HEI10-242000(1998)). With this arrangement, the adhesion of the carbon layer 6 to the mixture layer is increased by the carbon powder contained in the mixture layer, and the adhesion of the silver paste layer 60 to the mixture layer is increased by the silver powder contained in the mixture layer, whereby the ESR of the solid electrolytic capacitor is reduced.
Thus, the ESR is reduced by providing the carbon powder/silver powder mixture layer between the carbon layer 6 and the silver paste layer 60. The carbon layer 6 and the silver paste layer 60 are each formed by performing a coating operation once to several times and then performing a heat treatment, so that long production time is required for the production of the solid electrolytic capacitor 1. The formation of the carbon powder/silver powder mixture layer between the carbon layer 6 and the silver paste layer 60 further prolongs the production time, thereby increasing the production costs. Omission of either of the carbon layer 6 and the silver paste layer 60 may reduce the production time, but is not practical for the following reasons.
Where the cathode is produced by forming the silver paste layer directly on the solid electrolyte layer 3 as shown in FIGS. 5 and 6, for example, the contact resistance is increased with a smaller number of contacts between the solid electrolyte layer and surfaces of silver particles contained in the silver paste layer. Therefore, the ESR is increased even with the highly electrically conductive silver powder.
On the other hand, where the carbon layer is formed directly on the solid electrolyte layer 3 as shown in FIGS. 7 and 8, the contact resistance is reduced with a greater number of contacts between the solid electrolyte layer and surfaces of carbon particles contained in the carbon layer. However, carbon has a lower electrical conductivity than silver. Therefore, even if carbon having a relatively high conductivity is used, a desired level of electrical conductivity cannot be provided thereby to increase the ESR.
Therefore, a two-layer structure including the silver paste layer and the carbon layer is employed so as to electrically connect the solid electrolyte layer to the silver paste layer through the carbon layer having a reduced contact resistance in the cathode, thereby maintaining the ESR at a practical level.
In view of the foregoing, the inventor of the present invention has conceived the idea of reducing the production time while reducing the ESR as compared with the prior art. In the prior art, a silver paste for the conventional silver paste layer is prepared by kneading silver powder with a binder such as an epoxy resin or the like. The electrical resistance of the silver paste is measured, as shown in FIG. 9, by spreading the silver paste on a planer plate 66 and abutting electrodes 67, 67 with opposite ends of the resulting silver paste layer 65. This implicitly means that the paste was developed with the aim of reducing the resistance of the silver paste layer as plane direction measured (in X- and Y-directions). Therefore, the inventor conducted intensive studies in view of the fact that the conventional silver paste and the cathode 5 prepared by using the silver paste were developed without the aim of reducing an electrical resistance as measured along perpendicular direction of the capacitor element (in a Z-direction).